1. Field of the Invention
The present invention relates to reflective liquid crystal light valve systems and more particularly concerns transmission of light through certain layers of a multi-layer liquid crystal device.
2. Description of Related Art
The reflective liquid crystal light valve (LCLV) is a thin film multi-layer structure comprising a liquid crystal layer, alignment layers, a dielectric mirror layer, a light blocking layer and photosensitive layer, all sandwiched between two transparent electrically conductive electrode layers. In a typical reflective liquid crystal light valve projection system a high intensity polarized illumination or reading beam is directed through a quartz input window and through the liquid crystal layer to the dielectric mirror. In an optically addressed reflective liquid crystal light valve, an input image of low intensity writing light, such as that generated by a cathode ray tube, is applied to the photosensitive layer, thereby switching the electric field across the electrodes from the photosensitive layer onto to the liquid crystal layer so as to selectively activate the liquid crystal according to the intensity of input writing light received at different areas of the photosensitive layer. Linearly polarized illumination light from a high power light source, such as a xenon lamp, passes through the quartz input window, through one transparent electrode and through the liquid crystal layer and adjacent alignment layers to be reflected from the dielectric mirror. Light reflected from the mirror is polarization modulated by the liquid crystal in accordance with the spatial pattern of writing light information incident on the photo-responsive layer. Therefore, if a complex distribution of light, for example a high resolution low intensity input image from a cathode ray tube, is focused onto the photosensitive layer surface, the liquid crystal light valve converts the low intensity input image into a replica image which can be reflected for projection with magnification to produce a high brightness image on a viewing screen. Projection systems of this type are described in several U.S. Patents, including U.S. Pat. No. 4,650,286 to Koda et al for Liquid Crystal Light Valve Color Projector, U.S. Pat. No. 4,343,535 to Bleha, Jr. for Liquid Crystal Light Valve, U.S. Pat. No. 4,127,322 to Jacobsen, et al for High Brightness Full Color Image Light Valve Projection System, and U.S. Pat. No. 4,191,456 to Hong, et al for Optical Block for High Brightness Full Color Video Projection System.
In some reflective liquid crystal light valves an array of thin film transistors incorporated in the mirror acts as the image input. The transistors are selectively actuated to block the high intensity reading light in a selected spatial pattern which causes the liquid crystal material to change the polarization of the reflected high intensity reading light in a like spatial pattern.
Many reflective liquid crystal light valve systems, particularly those having high intensity reading light with a line spectral content, or quasi monochromatic light as well as highly monochromatic light (e.g. from a laser), exhibit highly undesirable interference fringes in the output of the projection system. The interference fringes are considerably less of a problem, being less prominent, when the reading illumination source is in the form of a broadband source, such as a xenon arc lamp. Accordingly, xenon arc lamps are commonly used as the illumination source in liquid crystal light valve projectors. However, these lamps have a number of disadvantages when compared with metal halide lamps. Xenon arc lamps are inefficient, they create safety hazards, have short life times, and require large power supplies. Moreover, the xenon arc lamp produces much of its output in the infrared spectrum, thereby producing less useful light and more undesired heat. Thus they are less efficient. In some arrangements intensity of the illumination source is limited by allowable temperatures of the liquid crystal light valve device.
A metal halide lamp, such as a mercury lamp for example, provides more light with less heat, as compared to a xenon lamp, and is preferable to the xenon arc lamp in many other respects. However, light produced by the metal halide lamp is less broadband than that of the xenon arc lamp and provides illumination in the form of a line spectrum. The light output of such lamps, in other words, is concentrated in relatively narrow bands of different wavelengths. This line spectrum output of the metal halide lamp greatly exacerbates the appearance of interference fringes.
Some liquid crystal light valves are employed in optical data processing applications where the reading illumination source is a laser. Because of the very narrow bandwidth of laser light, interference fringes become much more visible. In an attempt to minimize such interference fringes, where the light source is a laser, the system uses a liquid crystal light valve with exceedingly tight tolerances on the thickness of the liquid crystal layer. With presently available manufacturing techniques, only a small percentage of the manufactured devices can meet such tolerances. This results in a very low yield operation that dramatically increases light valve costs.
Accordingly, it is an object of the present invention to provide a reflective liquid crystal light valve system that avoids or minimizes above mentioned problems.